1.Field of the Invention
The present invention relates to a laser of a ring cavity (or resonator) type which is usable for optical interconnection, parallel data processing, large-capacity parallel optical transmission and so forth, and particularly to a laser, such as a semiconductor laser, having a three-dimensional ring cavity.
2.Description of the Related Background Art
Semiconductor lasers are well known as light sources which are usable in optical communication and optical recording, and various types thereof have been developed. Further, in recent years, there have been developed opto-electronic integrated circuits in which optical functional devices, such as a semiconductor laser, a photodetector, a modulator and an optical switch, are arranged on a common substrate, and the integration of arrays of semiconductor lasers suitably usable for parallel processing has also been studied. In respect of those integrated circuits, functional improvement of the semiconductor laser is strongly required, and especially a low-threshold semiconductor laser is a key device.
With an approach to such a low-threshold semiconductor laser, a so-called micro-cavity structure is known, whose cavity length is reduced to about a wavelength of light to increase a coupling rate of spontaneous radiation light to its oscillation mode. Devices as illustrated in FIGS. 1, 2A and 2B are known, as examples of such a laser. The device of FIG. 1 is a surface emitting semiconductor laser in which distributed Bragg reflectors 753 and 755 are provided above and under an active layer 751, respectively, a current can be injected into the active layer 751 through upper and lower electrodes 758 and 759, and its cavity is constructed perpendicular to the substrate 757.
The devices of FIGS. 2A and 2B are disc-type semiconductor lasers in which there are arranged discs 873 and 893 having diameters of about several microns and including circular and hexagonal active layers 875 and 895 above substrates 871 and 891, respectively in which and light is totally reflected at peripheral surfaces of those discs 873 and 893, which thus provide ring cavities in planes parallel to the substrates 871 and 891, respectively.
The surface emitting semiconductor laser as illustrated in FIG. 1, however, needs distributed Bragg reflectors with high reflectivity. When a GaAs substrate is used as a substrate, an AlAs/(Al)GaAs multi-layer mirror is ordinarily used as the reflector. In this case, however, more than twenty pairs of AlAs/(Al)GaAs are needed to obtain a sufficiently high reflectivity, and hence it takes much time to grow those pairs on a wafer. Further, when an InP substrate is used, it is necessary to etch the substrate with an active layer and the like grown thereon to form a cylindrical hole in the substrate and to deposit an SiO.sub.2 /Si or Al.sub.2 O.sub.3 /Si multi-layer in the hole by vacuum evaporation or sputtering. The process is hence complicated. In addition, it is difficult to further lower the reduced threshold since light propagating in directions parallel or slanting to the substrate is only weakly coupled to the cavity mode.
On the other hand, in the case of the disc-type semiconductor lasers, the circular disc is ordinarily formed by dry-etching a predetermined portion after the crystalline growth. Therefore, it is difficult to precisely form the circular periphery of the disc's side, and the side face of the active layer is likely to be damaged. In addition, it is also difficult in this case to further lower the reduced threshold since light propagating in directions parallel or slanting to the substrate is not strongly coupled to the cavity mode.
Regarding the case of FIG. 2B, there is indeed a method of forming the hexagonal structure by a selective growth using the face-orientation dependence of crystalline growth speed, and this method can improve the flatness of the side face, compared with dry etching. Also in this case, however, it is likewise difficult to further lower the reduced threshold because light propagating in directions parallel or slanting to the substrate is not strongly coupled to the cavity mode.
Furthermore, in semiconductor lasers which have thus far been developed, oscillation basically occurs in a linearly-polarized light mode, and a semiconductor laser capable of oscillating in a circularly-polarized or elliptically-polarized light mode has not yet been put into practical use.